The present invention relates to softball and baseball bats and more particularly relates to the use of supportive structural members to improve the bat""s impact response.
Tubular metallic softball and baseball bats are well known in the art. A familiar example is a tubular aluminum bat. Particular embodiments of inserts for tubular baseball and softball bats are known in the art. Eggiman""s U.S. Pat. No. 5,415,398 (Eggiman), which is incorporated herein by reference, concerns a softball bat, including an insert to provide structural support to the bat. The Background section of Eggiman discusses some prior designs produced in the continuing quest for bats with a better xe2x80x9csluggingxe2x80x9d capacity. Eggiman also discusses the desire to have bats with large elastic deflections, but without plastic deflection upon impact.
Eggiman discloses a tubular insert that fits within the frame of the bat, and is movable relative to the bat. In one embodiment the insert is separated from the frame by a gap that allows movement of the frame relative to the bat. Such an insert improves the impact response of the bat by increasing elastic deflection without allowing plastic deflection. The Eggiman softball bat is a considerable improvement over prior bat designs providing increased power transfer from the bat to the ball.
Nevertheless, the need for a bat with even better slugging capacity continues. It is desirable to improve the slugging capacity of bats without adding significant weight. This is particularly difficult in baseball bats where high impact forces require greater structural integrity within the bat to prevent unwanted plastic deformation. Prior baseball bats that provide this structural integrity typically added significant weight to the bat, did not allow sufficient elastic deformation within the bat, or decreased the diameter of the bat in the impact area to compensate for the added weight to the bat.
A further problem with prior bats is the size of the optimal hitting area or xe2x80x9csweet spotxe2x80x9d of the bat. The sweet spot is typically located near the center of the impact area of the bat. The performance of the bat declines considerably when a ball impacts the bat outside the sweet spot, for example near the end of the bat. Typically, such an impact results in greater vibrations being felt by the user and decreased energy transfer from the bat to the ball.
Composite materials have been used to form the structural components of bats to improve their performance. An example is U.S. Pat. No. 5,364,095 by Easton. Easton discloses a tubular metal bat that is internally reinforced with a fiber composite sleeve. The Easton sleeve consists of multiple fiber layers, with each layer having bi-directional woven fibers that are directed at 0 and 90 degrees relative to the axis of the bat. The sleeve is formed so that it is bonded to the aluminum frame and it presses outwardly on the frame to produce a pre-load stress of several thousand pounds per square inch. This design does not allow for sufficient elastic deformation of the external frame of the bat. Moreover, the properties of the Easton composite sleeve are essentially isotropic.
The present invention provides an improved insert, sleeve, or tubular member, for use with a baseball or softball bat that improves power transfer from the bat to the batted ball and that overcomes the shortcomings of the prior art. More specifically, the present invention provides a tubular member having anisotropic properties such that the tubular member is stronger in the direction of the greatest stress produced by an impact with a ball. By producing such anisotropic properties, less material, and therefore less weight, is required to produce the necessary stiffness and strength within the tubular member. Moreover, the impact response of the bat is improved.
In a preferred embodiment, the tubular member includes multiple layers of composite material. Each layer includes fibers that are directed substantially circumferentially about the tubular member. Since the largest component of stress within the tubular member produced by an impact with a ball is typically compressive hoop stress, which is directed substantially circumferentially about the tubular member, the circumferential layers are able to withstand the majority of the stress produced by an impact. The tubular member also includes non-circumferential layers that serve to hold the circumferential layers together and withstand the smaller stresses in other directions.
Also in a preferred embodiment, the tubular member is secured to the bat at each end of the tubular member. The intermediate portion of the tubular member is able to move freely relative to the tubular frame of the bat, thus allowing the tubular member to act as a leaf spring when the frame of the bat elastically deflects and impacts the tubular member.
The invention also provides a process for making the tubular member and a bat including the tubular member. The process includes positioning composite layers adjacent each other, such that each layer is tubular, and orienting at least one of the layers, such that unidirectional fibers supported within that layer extend substantially circumferentially. In one embodiment, the process includes wrapping the layers about a mandrel and subsequently curing the layers by heating the layers and applying pressure to the layers while they are heated.